Extrusion coating involves the process of extruding a molten film from a die and contacting this molten film with a substrate under pressure in the nip of two counter-rotating rolls or the like. Both of these rolls can be chill rolls, which by definition means temperature controllable rolls. Alternatively, one of these rolls can be a pressure roll, which has a deformable surface able to distribute pressure over an uneven surface. The pressure roll is in contact with the side not being coated, and the chill roll is in contact with the side being coated.
Extrusion coating of woven and non-woven polyolefin substrates involves several problems. Some examples of such problems are extruder surging, poor coating adhesion particularly to polypropylene, and unaesthetic hand for some coatings having a thickness greater than one mil. To decrease extruder surging, which is the non-uniformity of gauge or coating weight in the machine direction, and to increase adhesion of the extruded coating to polypropylene substrates, the melt flow rate of the extrusion coating is generally increased. However, to decrease undesirable neck-in which is the difference between the width of the web as it exits from a die and that width at the point of contact with a substrate, the melt flow and temperature of the extruded coating are generally decreased. In summary, to achieve a suitable extrusion coating for polyolefins and more specifically polypropylene, the range of temperatures and flow rates that are usable must resolve a variety of problems which have conflicting methods of solution.
Good adhesion with extrusion coatings of low density polyethylene to high density polyethylene fabrics requires extrusion temperatures of about 600.degree. F. in order to cause some melt oxidation. However, neither low density nor high density polyethylene alone can be made to adhere well to polypropylene fabrics regardless of process conditions. Surface primers for polypropylene fabrics have had limited success.
In U.S. Pat. No. 3,524,759 (1970) of R. L. McConnell et al., a process for curtain coating of flat sheets, paper, cardboard, articles of any size or shape, etc. is disclosed. The composition employed in that curtain coating process was a thermoplastic composition having a melt viscosity at 190.degree. C. of 5,000 to 125,000 cp. comprising:
(1) 40 to 99 percent by weight of a copolymer of 55 to 99 percent by weight of ethylene and 1 to 45 percent of at least one comonomer selected from the group consisting of:
(a) alkenyl alkanoates wherein said alkenyl portion contains 2-4 carbon atoms and said alkanoate portion contains 2-6 carbon atoms,
(b) alkyl acrylates, wherein said alkyl contains 1 to 8 carbon atoms,
(c) alkyl methacrylates wherein said alkyl contains 1 to 8 carbon atoms,
(d) acrylic acid,
(e) methacrylic acid, and
(2) 1 to 60 percent by weight of a crystallizable polymer of an .alpha.-olefin having 2 to 10 carbon atoms, said polymer having a melt viscosity at 190.degree. C. of 1,000 to about 20,000 cp. when the polymer is polyethylene and a melt viscosity at 190.degree. C. of 1,000 to about 400,000 cp. when the polymer is polypropylene or other higher polyolefin homo- or copolymer. In preferred embodiments of this invention, the melt viscosity of the coating composition is at least 15,000 cp. and the copolymer is ethylene/vinyl acetate.
McConnell et al. U.S. Pat. No. 3,524,759 does not disclose the use of similar coating compositions for an extrusion coating process nor the use of polyolefins and particularly polypropylene as coatable substrates. In the relevant art, curtain coating is a different and distinct coating process from that of an extrusion coating process. Each requires coating compositions having very different rheological properties. A principal limitation of curtain coating is that useful coating compositions must be sufficiently fluid to fall freely and sufficiently cohesive to present a continuous film to the substrate.